In a signal to Moscow of U.S. and NATO resolve, Army Gen. Curtis M. "Mike" Scaparrotti ended a two-day visit to Ukraine on Thursday in which he toured the combat positions of Ukrainian troops facing off against a combined Russian and separatist force.

On his second visit to Ukraine within three weeks, Scaparrotti, the dual-hatted Supreme NATO commander and head of U.S. European Command, went to eastern Ukraine, "where he visited Ukrainian military units involved in combat operations with Russians and their proxy forces" along what is called the "line of contact," EuCom said in a statement.

Scaparrotti also met with Ukrainian Minister of Defense Stepan Poltorak and Chief of the General Staff Viktor Muzhenko on the continuing training of Ukrainian forces by NATO and the supply of "non-lethal" aid by the U.S.

Russian news outlets took note of Scaparrotti's front-lines tour. The government-owned Sputnik news agency said that Scaparrotti's visit was focused on inspecting the performance of U.S. equipment supplied to Ukrainian troops.

The EuCom statement said Scaparrotti's visit underlined the U.S. and allied commitment to Ukraine's defense. "The United States continues to call for an immediate end to Russia's occupation of Crimea and for Russia to cease its destabilizing actions in Ukraine and honor its commitments in the Minsk agreements," it states.

The tour of the eastern combat zone came three weeks after Scaparrotti's last visit to Ukraine, during which he met with U.S. troops of the Joint Multinational Training Group-Ukraine at the International Peacekeeping and Security Center in Yavoriv, well away from the front lines.

During the November visit, Scaparrotti observed the training of Ukrainian forces by a multinational group including troops from the U.S., Poland, Lithuania, Canada and Britain.

Scaparrotti's latest visit to Ukraine came on the same day that a European Union meeting in Brussels agreed to extend sanctions on Russia for six months over the takeover of Crimea and the intervention in eastern Ukraine. Russia's annexation of Crimea in 2014 triggered the first round of sanctions.

The U.S. and its NATO allies have expressed increasing concerns over Russian threats against the Baltic states and the military buildup in the Kaliningrad Russian enclave bordering Lithuania.

At NATO headquarters in Brussels on Thursday, NATO Secretary-General Jens Stoltenberg said he had invited members of the NATO-Russia Council, the forum bringing together NATO ambassadors and Russia's top diplomat attached to the U.S.-led alliance, to a meeting in Brussels on Monday.

"When tensions run high, as today, it is even more important to have direct dialogue with Russia," Stoltenberg said.